I've installed LibUSB, a software that modifies your USB driver so that programs can access it easily. I didn't know that in Windows 7 you have to run it in compatibility mode for Windows XP, so now my Windows 7 can't access any USB port (including keyboard and mouse). Now I can't access my computer at all, and I don't have any PS2 connection in my computer.
I tried to run it in Safe Mode but it didn't change anything.
I tried to run a system restore from before the installation of LibUSB, but all the restore points give me an error: "The instruction at 0x73888f18 referenced memory at 0x00000004. The memory could not be read."
I also tried to run Hiren's Boot CD and remove all the files associated with LibUSB, but that didn't change anything.
So how can I remove LibUSB and access my computer again without formatting my hard drive? (I have important things on it)

There are many reports of issues with libusb and windows 7. I have also had this problem but cannot remember exactly how to resolve the issue. You should be able to resolve the issue without reinstalling or formatting your disk.

You need to uninstall the driver if possible by using F8 on startup to enter safe mode, then running the uninstall program for libusb.